The invention relates to printing on plastic and in particular to promotion of adhesion of printing inks to a plastic surface.
Surfaces of plastic products of all types) such as for example plastic shopping bags, place mats, and decals are printed for purposes of advertisement and decoration. However, there are many plastics to which common printing inks do not readily adhere. Often a plastic surface that is to be printed must be treated prior to printing so that inks that are used to print the su adhere properly to the surface. The treatment generally comprises coating the surface with a primer material that has a high affinity for both the plastic surface and the inks used to print the surface. The affinities of the primer for both the plastic and the ink bonds ink applied to the primer coated surface to the surface. Typically, the primer comprises or is mixed with volatile organic compounds potentially dangerous to human health that are released into the atmosphere during application of the primer to the surface. In order to protect people present in work areas where the primer is being used from exposure to the compounds, substantial resources have to be invested to properly ventilate the work areas and to monitor concentrations of the compounds in the air in the work areas.
An aspect of some preferred embodiments of the present invention relates to providing a primer, hereinafter referred to as a xe2x80x9cpollution free primerxe2x80x9d, for bonding a toner ink to a plastic surface that does not release organic compounds into the air.
An aspect of some preferred embodiments of the present invention relates to providing a pollution free primer that adheres with appropriate forces to the plastic surface and the toner so that the toner can be applied to the surface using an offset process.
An aspect of some preferred embodiments of the present invention relates to providing a primer that when applied to the plastic surface flows and wets the surface to provide a substantially uniform coating on the surface.
In a preferred embodiment of the present invention the primer comprises an aqueous solution of a first material having a strong affinity for the plastic surface mixed with a second material having a strong affinity for the first material and the toner. The affinities of the primer components for each other and for the plastic and the toner are such that the toner is efficiently transferred to the plastic from a blanket used in an offset printing process. Preferably the first and second materials form relatively strong bonds to each other at least when dried. Hereinafter, the first material is referred to as a xe2x80x9cplastic bonding-agentxe2x80x9d and the second material is referred to as a xe2x80x9ctoner bonding-agentxe2x80x9d.
In a preferred embodiment of the present invention, the plastic bonding-agent is polyethylenimine (PEI) and the toner bonding-agent is a dispersed phase of MP4990, which is an aqueous dispersion of copolymer of polyethylene and acrylic acid, manufactured by Michelman. Preferably, the primer is mixed with a surfactant that enhances wetting of the plastic surface by the primer and promotes formation of a uniform coating of the primer on the plastic surface.
The inventors have found that a thin substantially uniform coating of the primer can be applied to a side of a sheet or web of plastic using a gravure printing process. The primer adheres well to the plastic and the primer coated surface can be printed with liquid toners, such as for example those described in U.S. Pat. No. 5,407,771 to Landa et al., the disclosure of which is incorporated herein by reference. Preferably, the toners are ElectroInk(copyright) produced and sold by Indigo N. V. of the Netherlands such as ElectroInk(copyright) EI-Mark 3.0 and EI-Mark 3.1. Other toners can be used.
There is therefore provided in accordance with a referred embodiment of the present invention a method for preparing a plastic surface for printing with a toner comprising: forming a layer of an aqueous mixture comprising a first material and a second material on the surface, wherein the first material has an affinity for the plastic and the second material has an affinity for the first material and the toner; and drying the layer.
Preferably, the first material is polyethylenimine. Preferably, the second material is a material chosen from the group of materials comprising: polyethylene acrylic acid copolymer, styrene acrylate copolymer and styrene-butadiene. Peferably, the weight of the first material plus the weight of the second material is between about 2% and about 10% of the weight of the mixture.
Additionally or alternatively, the ratio of the weight of the second material to the weight of first material in the mixture is between 1 and 5. In some preferred embodiments of the present invention, the ratio of the weight of the second material to the weight of the first material in the mixture is substantially equal to 1. In some preferred embodiments of the present invention, the ratio of the weight of the second material to the weight of the first material in the mixture is substantially equal to 2. In some preferred embodiments of the present invention, the ratio of the weight of the second material to the weight of the fist material in the mixture is substantially equal to 3.
In some preferred embodiments of the present invention, the second material is polyethylene acrylic acid copolymer. In some preferred embodiments of the present invention, the second material is styrene acrylate copolymer. In some preferred embodiments of the present invention, the second material is styrene-butadiene.
According to some preferred embodiments of the present invention, the method comprises adding a surfactant to the mixture. Preferably, the surfactant is chosen from the group of materials comprising: 2,4,7,9-tetramethyl-5-decyn-4, 7-diol; 2,4,7,9-tetramethyl-5-decyn-4, 7-diol+3.5 moles ethylene oxide; and sodium dodecyl sulfate.
In some preferred embodiments of the present invention, the surfactant is 2,4,7,9-tetramethyl-5-decyn-4, 7-diol. In some preferred embodiments of the present invention, the: surfactant is 2,4,7,9-tetramethyl-5-decyn-4, 7-diol+3.5 moles ethylene oxide. Additionally or alternatively, after addition of the surfactant, the mixture comprises between 1% and 1.5% by weight surfactant.
In some preferred embodiments of the present invention, the surfactant is sodium dodecyl sulfate. Preferably, after addition of the surfactant, the mixture comprises between 2% and 2.5% by weight surfactant.
In some preferred embodiments of the present invention, the first material is dissolved in water. Preferably, the solution of the first material comprises between 2% and 10%/ by, weight of the first material.
In some prefer embodiments of the present invention, the second material is dispersed in water. Preferably, the aqueous dispersion of the second material comprises between 2% and 10% by weight of the second material.
In some preferred embodiments of the present invention, forming the layer comprises preparing the mixture in accordance with a procedure comprising: preparing an aqueous solution of the first material; preparing an aqueous dispersion of the second material; and mixing together the aqueous solution and the aqueous dispersion to prepare the mixture.
In some preferred embodiments of the present invention, forming a layer comprises forming a layer of an aqueous mixture that does not release organic compounds into the air during drying thereof.
In some preferred embodiments of the present invention, forming a layer comprises forming a layer having a weight per square meter of plastic between 0.10 and 0.20 grams after the layer is dry. In some preferred embodiments of the present invention, forming a layer comprises forming a layer having a weight per square meter of plastic between 0.125 and 0.175 grams after the layer is dry. In some preferred embodiments of the present invention, forming a layer comprises forming a layer having a weight per square meter of plastic between 0.135 and 0.165 grams after the layer is dry. In some preferred embodiments of the present invention, forming a layer comprises forming a layer having a weight per square meter of plastic substantially equal to 0.15 grams after the layer is dry.
In some preferred embodiments of the present invention, forming a layer of the mixture on the surface comprises applying the mixture to the surface using a gravure roller.
In some preferred embodiments of the present invention forming a layer of the mixture on the surface comprises applying the mixture to the surface using a gravure roller.